In U.S. Pat. No. 3,366,095 which issued Jan. 30, 1968 for "Control System and Warp Switch," there is disclosed a boiler control circuit. One element of that circuit comprises an electrode normally immersed in the boiler water. As long as an electrical water path exists between the electrode and ground, the circuit functions normally. In the absence of such a path, the circuit prevents the burner from igniting.
While the patented circuit represents an important advance over the prior art, the water level electrode portion can be defeated if a short develops between the electrode and ground. The circuit is then unable to distinguish between such a short and a water path. Accordingly, it is a primary object of the present invention to provide a test sub-circuit for differentiating between a water path and a short circuit. Other objects, features, and advantages will become apparent from the following description and appended claims.